Are there any ecological systems that would survive on other planets? What would happen if we 'seeded' other planets in the solar system with potentially compatible ecological systems?

I'll just copy my response to one of the other top level responses in the thread, and throw in a cool paper related to this at the end.

There is plenty of life that doesn't require sunlight--huge quantities of it. Possibly the majority. Subsurface bacteria make up an indeterminate but large fraction of all the biomass on Earth. To be sure, some of them ultimately rely on products of photosynthetic life, but many don't. There are bacteria beneath our feet that eat rocks, or absorb the energy released by radioactive decay, or just straight up slurp electrons from wherever they're sitting. And there are many bacteria living off the energy from vents on the seafloor. A good fraction of life on Earth could make a living on Mars or in the seas of Enceladus or Europa. All you need is a redox gradient, and there is almost certainly a bacteria that has evolved to exploit that gradient

We remain blissfully ignorant of the vital inputs provided by organisms we treat with contempt. Recently we had come to appreciate "extremophiles": organisms which endure and even thrive under conditions normally we would exclude harbouring life.

There are a gazillion exo-planets, at least a few of which gravitate in the goldilocks zone. On odds. one might expect life, in some form, to have prospered there. There just is NO evidence.

There are bacteria which can survive in space. That's not the same as living, it's just stasis.

All Earth life requires water, generally a lot of it. And virtually all the ecosystems also require sunlight. (A few ecosystems exist in extreme conditions such as near volcanic vents, or sulphurous lakes, but they are even more fragile than the surface life, when removed from their very specific environment.)

There is nowhere else in the solar system with a combination of water and sunlight. We believe there are under-ice oceans, and even some surface ice, but no liquid water where sun is also available. (And they're much further away from the Sun anyways.)

So no, there is no Earth life we can transplant to make an ecosystem elsewhere that we know of. At best we might find or make a few species of microbes that could thrive on Mars (and maybe in one of those oceans, if we knew anything about the chemistry). But I don't think a planet wide algae bloom counts as an ecosystem, it's just pollution.

Enceladus has directly sampled chemistry because Cassini flew through a geyser plume. Water, salt, hydrogen sulfide. Anaerobic bacteria would be totally fine here and constitute an ecosystem, although not a very showy one. NASA is very afraid of future missions accidentally adding bacteria if not sterilized enough.

This simply isn't true. There is plenty of life that doesn't require sunlight--huge quantities of it. Possibly the majority. Subsurface bacteria make up an indeterminate but large fraction of all the biomass on Earth. To be sure, some of them ultimately rely on products of photosynthetic life, but many don't. There are bacteria beneath our feet that eat rocks, or absorb the energy released by radioactive decay, or just straight up slurp electrons from wherever they're sitting. And there are many bacteria living off the energy from vents on the seafloor. A good fraction of life on Earth could make a living on Mars or in the seas of Enceladus or Europa. All you need is a redox gradient, and there is almost certainly a bacteria that has evolved to exploit that gradient

You're right, I missed the last bit, sorry about that, but I would argue that you could absolutely have purely microbial ecosystems full of these types of creatures--I mean, that's what Earth had for a good couple billion years

What do you mean? The GOE took place hundreds of millions of years after microbes started hanging out here. The formed communities for a very long time before any waste-product poisoning, and even then, there's no actual evidence of mass extinction; it's just assumed to have led to that

To me, an ecosystem must be self sustaining indefinitely (with an external energy source), or it's not really a system, it's just some stuff which is alive for now.

Granted, that's a pretty strict definition, and maybe not useful in this context. But if the intention was to seed other planets with life, I don't think sending a handful of species of bacteria really accomplishes that goal by itself. Might as well just send artificial terraria.

how do you define an ecosystem then? How many different species does it need to qualify?

The way I see it, if you have algae, something that eats them and then dies to become food for another type of life that shits out algae food, you have an ecosystem. Not a very fancy one, but nevertheless.